At the present time, to make means available for protecting a sensitive area from any intrusions constitutes a challenge which is both difficult and poorly mastered. This is because, depending on the nature of the terrain constituting the area in question, the desired degree of selectivity and the permanent or temporary nature of the protection-installed, the solution to be implemented may take substantially different forms. Therefore, depending on the type of protection desired, it is necessary to determine the most appropriate type of protection systems from those available on the market. However, the proposed solutions at the present time are few in number and in general are not capable of simultaneously satisfying all the criteria defining the desired type of area protection.
A first known type of protection consists in installing a perimeter surveillance system comprising distributed sensors placed along a line defining the entry into the sensitive area, these sensors having the main role of detecting when this line is breached by one or more intruders. As is known, depending on the circumstances it is possible to use simple seismic sensors or, more sophisticatedly, barriers based on infrared sensors.
The detection elements used are generally defined by taking into account the temporary or permanent nature of the barrier installed and also the logistic criteria such as, for example, the ease of transporting these elements on a person's back or on a light vehicle, or else their speed of implementation, in order to form the desired protection barrier.
Advantageously, such systems are simple to operate, but they have a number of drawbacks.
Firstly, it should be noted that the protection offered by this type of protection is very generally limited to signalling an intrusion at the very instant when the breaching of the barrier thus formed occurs. In the most favourable case, this signalling is accompanied by information of varying precision, relating to the place where the barrier was breached. Therefore, these systems do not allow an anticipated signalling procedure to be carried out, one associated with the sensitive area being approached by possible intruders.
Secondly, it should also be noted that, after an intrusion has been detected, this type of protection generally no longer provides any information regarding the advance of the intruder in the sensitive area so that, unless there is a very rapid reaction, the search for the intruder(s) entails a complete search of the area.
Thirdly, it should also be noted that with regard to the particular case of systems using optical detectors or infrared detectors to form a perimeter barrier, the effectiveness of the barrier is generally worse when the atmosphere becomes too humid, so that detection is no longer automatically guaranteed.
Another known type of protection consists in installing a centrally located surveillance system based on the use of a short-range surveillance radar operating for example in the S band or in the X band and designed to keep the entire area to be protected under surveillance. The use of such a system has many advantages.
Firstly, it makes it possible both to detect the breaching of the line defining the boundary of the sensitive area and to monitor the space constituting this area.
Furthermore, such a radar system is largely insensitive to the weather conditions. It also makes it possible not only to detect an intrusion and to track the progress of the intruder, but also to know the intruder's rate of progress. It is then advantageously possible, by performing a qualitative analysis of the signal that has given rise to the indication of an intrusion, to determine the nature of the target and for example to determine whether the target is of the pedestrian or vehicle type. It is thus possible to determine, all at the same time, the nature of the intruder, the position of the point of intrusion and the movement of the intruder within the sensitive area.
In operational terms, the use of such a system also makes it possible for the work of monitoring the protected area to be advantageously simplified, notably by mobilizing only a single operator. Nevertheless, the use of such a solution does have drawbacks too:
it is generally expensive because of the degree of sophistication of the radar used;
owing to the use of a short-range surveillance radar operating in the S or X band, it proves to be inoperable whenever the space covered has elements that may form an obstacle to the direct propagation of the transmitted waves, because of the very wavelengths used. This is in particular the case if the presence of vegetation is noted in the area to be protected. In such a case, if the sensitive area includes wooded areas, the coverage of these wooded areas is not ensured so that an intruder can profit from their use in order to penetrate the sensitive area and progress therein without being immediately detected.